In recent years, there have been known methods for judging whether credit cards and brand goods or the like are forged, for preventing the forgery of credit cards or the like and preventing the forgery of brand goods. As one type of method, for example, there has been known an optical reading device. In the optical reading device, marks or the like are printed using a phosphor-containing ink unobservable to the naked eye to form a latent image mark. The latent image mark is irradiated with visible light or infra-red light to excite the phosphor. The infra-red light emitted from the phosphor and hardly observed by the naked eye are received, and the latent image mark is detected.
Since the latent image mark for judging what is real and what is false is not easily visible to the naked eye according to this method, a forger rarely prints this latent image mark, and cards and articles forged or altered can be reliably detected. Also, since only a genuine card manufacturer and an article manufacturer know the contents of the latent image mark, it is very difficult to forge or alter the card or the like.
As such a phosphor, there have been known many phosphors which contain neodymium (Nd) and ytterbium (Yb), for example, Ca10(PO4)6F2:Nd,Yb, Ca8La2(PO4)6O2:Nd,Yb, YAlO3:Nd,Yb, Y3Al5O12:Nd,Yb, (Y,La,Lu)PO4:Nd,Yb, (Nd,Yb)P3O9, (Nd,Yb)P5O14, (Li,Na,K)(Nd,Yb)P4O12, K3(Nd,Yb)P2O8, Na(Nd,Yb)(WO4)2, Na5(Nd,Yb)(WO4)4, Na(Nd,Yb)(MoO4)2, Na5(Nd,Yb)(MoO4)4, Na2(Nd,Yb)Mg2(VO4)3, (Al,Cr)3(Nd,Yb)(BO3)4, Na5(Nd,Yb)((Si,Ge)O3)4, Na3(Nd,Yb)(Si,Ge)2O7, (Nd,Yb)MgAl11O19 and a material prepared by replacing a part of neodymium (Nd) and ytterbium (Yb) having the above composition with at least one element selected from, for example, Y, Sc, La, Ce, Gd, Lu, Ga, In, Sb and Bi as the element which can take trivalence and has no absorption in a wavelength range of 800 nm to 1000 nm (for example, see Patent Documents 1 to 4).
Of these phosphors, for example, an Na5(Nd,Yb)(MoO4)4 phosphor has often been used for preventing forgery in view of emission intensity, sensitivity and conformity with a photoreceiver.    Patent Document 1: Japanese Laid-Open Patent Publication No. 54-33634 (pages 2, 3)    Patent Document 2: Japanese Laid-Open Patent Publication No. 54-100991 (pages 1-3)    Patent Document 3: Japanese Laid-Open Patent Publication No. 3-288984 (page 2)    Patent Document 4: Japanese Patent No. 3438188 (pages 1, 2)